Always thought of as addictive, nicotine is now shown to induce a progressive and long-lasting increase of sensitivity in the brain reward systems, reports an online publication from the journal Neuropsychopharmacology.
In 2003, 45.4 million adults in the United States were smokers. An estimated 70% of these said they wanted to quit. Previous studies have concluded that nicotine is an addictive drug on par with heroin and cocaine and this work may help us understand why it is so hard to kick the habit.
In the first study of this kind, researchers studied the "pleasure center" of the brain or the posterior lateral hypothalamus, and found that nicotine caused the brain to register an enhanced reward value long after intake. This differs from other drugs of abuse where there is usually a decrease in brain reward systems.
Paul J. Kenny and Athina Markou found that rats voluntarily consumed nicotine to increase the reward sensitivity in their brain. With a relatively low dosage, administered just one hour a day, the brain is alerted that nicotine is pleasurable and that this feeling is long lasting. This long-lasting effect may be linked to the addictive properties of nicotine and why it is not so easy to quit smoking - because your brain remembers how good the last cigarette was.
Future research will focus on whether this enhanced sensitivity is reversed with higher nicotine dosages and whether this leads to depression in chronic smokers. Follow up studies will also investigate the potential role that nicotine-induced reward hypersensitivity may play in perpetuating the nicotine habit and the side effects of quitting smoking. One example for consideration is whether reward hypersensitivity could increase the appetitive value of food, thereby contributing to the weight gain seen in former smokers.
Paul J. Kenny (The Scripps Research Institute, Jupiter, FL, USA)
Abstract available online.
(C) Neuropsychopharmacology press release.
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